The invention relates to a method for verification of the operability of two or more identical zener diodes which are connected in parallel with one another and are connected to a solenoid coil.
Electrically operated hydraulic or pneumatic valves control a hydraulic or pneumatic working medium depending on the magnitude of an electrical input signal. The electrical input signal is supplied to a solenoid coil, which deflects an armature as a function of the current flowing via the solenoid coil. If valves such as these are used in an explosion-hazard environment, particular safety regulations apply. These regulations require, inter alia, that, in the event of a discontinuity in a line leading to the solenoid coil, the induction voltage that is produced in this case is limited to a safe value. The aim of this is to prevent spark formation, which could cause explosive gases to explode. Two or three zener diodes are connected in parallel with a solenoid coil in order to limit the induction voltage that occurs in the event of a cable discontinuity, in accordance with the applicable regulations for explosion proof equipment. In order to limit the induction voltage irrespective of its polarity, two zener diodes are used, for example, connected in series in opposite senses. As an alternative to this, bidirectional zener diodes may also be used, which correspond to two single zener diodes, connected in series in opposite senses. The zener diodes are arranged physically as close as possible to the solenoid coil. In accordance with the safety regulations, the parallel circuit that is formed from the zener diodes and the solenoid coil is additionally encapsulated by means of an insulating compound. The encapsulated circuit forms an electrical two-pole network, from which only the two connecting lines lead to the exterior. Only the two connecting lines of the two-pole network are therefore available as measurement points for testing the operability of the zener diodes which are connected in parallel with the solenoid coil. It is not possible to test the operability of the zener diodes individually after they have been soldered into the circuit arrangement and have been encapsulated.